There has been much past effort aimed at producing a silver halide photographic material which has high photographic speed, is subject to little fogging and suffers little change in its photographic performance during storage.
For example, it has been known for a long time that the use of organic thioethers as silver halide solvents or chemical sensitizers, etc. in the manufacture of silver halide photographic emulsions improves photographic speed.
U.S. Pat. Nos. 3,271,157, 3,531,289, 3,574,628 and 4,057,429, for example, disclose techniques for manufacturing so-called monodisperse silver halide photographic emulsions in which the silver halide grains are of a uniform size by introducing organic thioether compounds in the precipitation stage or at the time of physical ripening. (Hereinafter, silver halide photographic emulsions will be referred to simply as "emulsions").
Also, techniques for increasing the photographic speed of emulsions by introducing organic thioether compounds at the time of chemical ripening in emulsion manufacture or immediately prior to coating have been disclosed, e.g., in U.S. Pat. Nos. 2,521,926, 3,021,215, 3,038,805, 3,506,443, 3,057,724, 3,062,646, 3,574,709, 3,622,329 and 3,625,697.
Further, a technique for increasing the photographic speed of emulsions by using macrocyclic ether compounds containing selenium atoms has been disclosed in U.S. Pat. No. 4,782,013.
There has also been disclosure, in JP-A-53-57817 (the term "JP-A" as used herein means an "unexamined published Japanese patent application"), of a technique for increasing the photographic speed of emulsions by the use of tellurium compounds which contain tellurium atoms and, preferably tellurium compounds which have substituted aromatic rings such as bis-(p-ethoxyphenyl) telluride, at the time of silver halide grain formation.
Although a variety of thioether compounds such as those noted above have been considered to date for the purpose of suppressing the occurrence of fogging while increasing photographic speed, none of these thioether compounds gives fully satisfactory effects.
The organotelluroether compounds of JP-A-53-57817, which discloses the use of telluroether compounds in place of thioether compounds, do indeed have speed enhancement effects. But their effects are still insufficient. Further, these tellurium compounds are unstable in light, heat and oxygen; and it is not easy to synthesize them.
U.S. Pat. No. 4,782,013 discloses compounds which have selenium atoms introduced in place of or in addition to sulfur atoms. However, these compounds are expensive for practical purposes since they are macrocyclic and synthesis of such compounds with good purity is difficult and yields low. In addition, fogging is liable to occur with these compounds, especially in color development.
Another common practice is to use a sensitizing dye to subject silver halide emulsions to so-called spectral sensitization consisting of sensitization up to a wavelength region in which the silver halide itself is not photosensitive.
For example, as may be seen in U.S. Pat. No. 3,506,443, it is known that use of a number of the abovenoted thioether compounds increases the spectrally sensitized speed of green-sensitive silver halide emulsions in which benzoxacarbocyanine or benzimidazolocarbocyanine dyes are employed, but results are still unsatisfactory even in this case.